This invention relates to processes for treating effluents obtained from commercial plants, and more particularly, it relates to treatment of effluents obtained during the production of acrolein and acrylic acid by the gas phase catalytic oxidation of propylene or the production of acrylic acid by the gas phase catalytic oxidation of acrolein.
The Chemical Oxygen Demand, or COD, is one of the criteria for evaluating the degree of pollution of water, as described in detail by M. Dore, N. Brunet and B. Legube in the review "Tribune Cebedeau", Vol. 28, No. 374, pp. 3-11 (1975), and is determined according to the standard AFNOR NF T 90-101 of 1971.
It is well known that acrolein and acrylic acid are generally prepared by the gas phase catalytic oxidation of propylene, and that these reactions require the presence of large quantities of water vapor in the gaseous reaction mixture. This results in the formation of fairly large quantities of dilute aqueous solutions, after condensation of such reaction mixture. Following separation of the main products, there remain undesirable aqueous effluents containing organic compounds in amounts of one to five percent by weight. These consist mainly of acrylic acid, acetic acid, and acrolein, together with other by-products such as propionaldehyde, acetone and formaldehyde, in smaller quantities.
The dilution, and volume, of these effluents are such that it is not economically practicable to destroy them by incineration. Moreover, when the content of organic compounds is high, of the order of 10,000 to 50,000 mg/L, expressed in terms of the Chemical Oxygen Demand (COD), for example, which also corresponds to a high Biochemical Oxygen Demand (the BOD is another criterion for evaluating the degree of pollution of water, and is also described in the above-mentioned article by M. Dore), the use of biological treatment methods, such as activated sludges, bacterial beds, or deep wells, for these effluents results in such high investment and conversion costs as to make these processes unsuitable for use.
The same applies to the recovery processes which attempt to make use of the organic derivatives contained in these effluents, such as the liquid-liquid extraction described, for example, in U.S. Pat. No. 3,816,524 for the concentration of an aqueous solution containing 1.9% by weight of acrylic acid, or in the article by R. W. Helsel in the review "Chemical Engineering Progress", Volume 73, (No. 5), pp. 55-59 (1977). Moreover, the large number of organic derivatives which comprise this waste water rules out the use of other processes such as electrodialysis, as described, for example, in Japanese Pat. No. 48-7610 of Mar. 7, 1973.
The detoxification of waste water from the production of acrylates by catalytic oxidation on pyrolusite, or on copper- or chrome-based catalysts, in the vapor phase, yields high degrees of purification, according to the studies made by V. S. Saltanov reported in "Khim. Prom.", Volume 44, No. 1, pp. 32-33 (1968). Moreover, the gaseous mixture obtained from this operation, consisting essentially of air and vapor, can be recycled into the oxidation stage of the propylene or acrolein, as described in Japanese Pat. Nos. 75-108,208 and 76-103,644 filed on Feb. 7, 1974 and Mar. 10, 1975, respectively. However, these processes consume energy and, in spite of everything, require an incineration phase for the destruction of the concentrated condensates.